The present invention relates to an environmental friendly vehicle. More particularly, the present invention relates to a hybrid electric vehicle (HEV).
The HEV of the present invention uses an engine in combination with an electric motor. An energy storage device is also used to store energy for driving the electric motor. The engine, preferably in conjunction with a generator (for series drive embodiment or without for a parallel embodiment), and the energy storage device work in combination to provide energy for powering the vehicle motor. A series HEV uses an engine with a generator (APU/PPU) to supply electricity to the motor and the energy storage system. A parallel HEV has a direct mechanical connection between the engine and the wheels. The use of electric power substantially cuts down on chemical emissions and vastly improves fuel economy. Although HEVs have been previously known, the HEV technology of the present invention provides significant advantages of providing a viable HEV technology that allows for a high performance HEV with a unique power management and distribution system.
The preferred embodiment of the present invention is based on the assembly and modification of hardware components and incorporation of control logic to produce a hybrid vehicle drive system that can utilize capacitive energy storage devices without addition of buck-boost or other similar electronic hardware between capacitor and electrical drive components to convert variable voltage power to fixed voltage power. The present invention is applicable to both series and parallel hybrid drive configurations and can utilize conventional batteries, flywheels, as well as ultra-capacitors.
Traditional electric hybrid vehicle drive systems have been set up using batteries for storage. Batteries are designed to operate at or near constant voltage as they are discharged and charged. Battery hybrid components are sized to operate at this nominal voltage. Control strategies for the Auxiliary Power Unit (APU), also referred to as the Primary Power Unit (PPU), are designed to hold the voltage within narrow values or set-points near the nominal voltage. When one tries to replace batteries directly with capacitors, the battery control system strategy, which is designed to maintain the voltage level, cannot take full advantage of capacitor storage. This under-utilization results in poor fuel economy, and for undersized APU/PPUs, poor vehicle performance will result.
To store energy in a capacitor, voltage must be allowed to vary up and down. The greater the variation, the more energy can be stored and extracted. Electrical components designed for constant voltage (battery) operation cannot deliver rated power when operated below narrow voltage parameters or set-points. These components will also overheat if operated at low voltage for extended periods. Control strategies set to protect these components will cut back output, reducing vehicle performance or cause a safety trip out. When voltage begins to rise above the nominal set-point, as during regenerative braking, the battery system control strategy tapers back APU/PPU output and reduces regeneration effect. If voltage was allowed to rise as would be needed to properly charge a capacitor bank, the control system will cause an over-voltage trip out, or if left unchecked, will damage components.
Even if the control over-voltage trip is removed, and components were sized to take the over-voltage condition, typical battery-type traction inverter drives produce unstable performance. This is usually because inverters set up for battery systems are not equipped with the control logic and high-speed data sampling needed to deal with the transient current spikes developed by low-inductance motors operating at low motor speeds and at higher-than-rated voltage. This is the condition that exists with a fully charged capacitor bank when a vehicle is starting up from rest.
Furthermore, constant voltage APU/PPUs will not deliver power to the traction drive when voltage is above the nominal set-point. This strategy does not allow it to share part of the initial load when accelerating the vehicle until voltage is near nominal. An optimally sized capacitor bank will become depleted before the acceleration event is completed.
This situation would require the APU/PPU to operate in an uneconomical peaking mode to finish the acceleration cycle or require a larger and proportionally costly and heavier capacitor bank. Conversely, during braking, a constant voltage APU/PPU will work to add power to the storage system. In an optimally sized capacitor bank, the capacity for capturing the braking energy would be reduced. Again, this would require a larger capacitor bank to accommodate this situation.
One solution to overcoming the variable voltage requirement for capacitor storage is to install an electronic device between the drive system set up for batteries and the capacitor bank. This device, usually of the buck-boost design, would convert the variable voltage power required to take advantage of the capacitor storage to the near constant voltage needed by the battery traction system. In other words, make the capacitor look like a battery. This solution adds expense and complexity to the system and lowers the efficiency of a capacitor storage system.
In the present invention, components and control strategies have been designed to allow for a wide fluctuation in voltage without performance loss or nuisance trip outs. APU/PPU performance is optimized with the least amount of capacitor bank requirement. The present system does not require additional devices between the drive and capacitor bank. In other words, the system electric bus is preferably connected to the ultracapacitor so that the electric bus voltage equals the capacitor voltage. The capacitor voltage varies directly with the variance of the electric bus voltage.
The preferred embodiment of the present invention is comprised of four major components:
One or more low inductance traction motor(s), capable of delivering rated torque and power at the low voltage set-point;
One or more traction inverter(s) capable of delivering rated power at the low voltage set-point. Components sized to operate at the high voltage set-point. Control set up to eliminate instability at high voltage when using a low inductance motor.
One or more APU/PPUs, comprised of a generator for a series embodiment, powered from an engine of any variety. This APU/PPU is designed to deliver rated power between high and low voltage set-points, peak vehicle power when energy in capacitor bank is depleted, and average power during acceleration. The engine can be mechanically connected in a parallel hybrid configuration and utilize the induction motor as the generator.
One or more capacitor bank(s) sized to deliver traction power above average requirement for accelerating the vehicle to rated speed and capturing braking energy from regeneration.
The control strategy of the preferred embodiment of the present invention utilizes the following:
Input parameters used in the control calculation preferably include engine speed, motor speed, vehicle speed, temperature, electrical bus voltage, motor current, generator current, positive or forward (acceleration) command, direction, and negative (deceleration) command. Other inputs may also be included depending on application.
Control output commands preferably include motor torque, generator voltage and current, engine speed, engine power, and shift command for transmission equipped vehicles. Other outputs may be included depending on application.
In the preferred embodiment of the present invention, the electric vehicle power management system is comprised of:                an electric motor;        an auxiliary power unit (APU) electrically coupled to the electric motor;        an ultracapacitor electrically coupled to the electric motor and the auxiliary power unit; and        where the auxiliary power unit and the ultracapacitor provide energy to the electric motor for powering said vehicle;        a power management controller is programmed to control the auxiliary power unit to vary output power to maintain the ultracapacitor between a predetermined high voltage set-point and a predetermined low voltage set-point;        where the power management controller runs the output of the auxiliary power unit up to the predetermined average power level when the ultracapacitor is at a predetermined range between the high and low voltage set-points, the range having a low threshold point and high threshold point;        where the power management controller is adapted to increase the output of the auxiliary power unit when the energy level of the ultracapacitor falls below the low threshold point of the range; and        wherein the power management controller is adapted to decrease the output of the auxiliary power unit when the energy level of the energy storage system reaches the high threshold point of the range.        
In the preferred embodiment, the power management controller maintains the system within a predetermined vehicle speed to capacitor voltage ratio by controlling the output power of the auxiliary power unit.
In addition to the features mentioned above, objects and advantages of the present invention will be readily apparent upon a reading of the following description.